U.S. patent application number 13/482154 was filed with the patent office on 2013-01-03 for optical coherence tomographic imaging apparatus, optical coherence tomographic imaging method, program for executing the optical coherence tomographic imaging method, and storage medium having the program stored thereon.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Ryoji Kurosaka.
Application Number | 20130003074 13/482154 |
Document ID | / |
Family ID | 47390360 |
Filed Date | 2013-01-03 |
United States Patent
Application |
20130003074 |
Kind Code |
A1 |
Kurosaka; Ryoji |
January 3, 2013 |
OPTICAL COHERENCE TOMOGRAPHIC IMAGING APPARATUS, OPTICAL COHERENCE
TOMOGRAPHIC IMAGING METHOD, PROGRAM FOR EXECUTING THE OPTICAL
COHERENCE TOMOGRAPHIC IMAGING METHOD, AND STORAGE MEDIUM HAVING THE
PROGRAM STORED THEREON
Abstract
In order to perform accurate evaluation for enhanced depth
imaging (EDI) in which a tomographic image of a retina has a low
luminance value in large part, provided is an optical coherence
tomographic imaging apparatus for acquiring an image of an object
to be inspected by irradiating the object to be inspected with
measuring light and causing return light from the object to be
inspected to interfere with reference light, the optical coherence
tomographic imaging apparatus including: a unit for setting an
imaging parameter of the image; a unit for selecting an image
quality evaluation index in accordance with the imaging parameter;
a unit for acquiring an image characteristic amount from the
acquired image in accordance with the set imaging parameter; and a
unit for evaluating image quality of the image based on the image
characteristic amount and the image quality evaluation index.
Inventors: |
Kurosaka; Ryoji; (Tokyo,
JP) |
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
47390360 |
Appl. No.: |
13/482154 |
Filed: |
May 29, 2012 |
Current U.S.
Class: |
356/479 |
Current CPC
Class: |
G01B 9/02091 20130101;
G01B 9/02067 20130101; G01B 9/02004 20130101; G01B 9/02 20130101;
G01B 9/02064 20130101 |
Class at
Publication: |
356/479 |
International
Class: |
G01B 9/02 20060101
G01B009/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 2011 |
JP |
2011-143919 |
Claims
1. An optical coherence tomographic imaging apparatus for acquiring
a tomographic image of an object to be inspected based on combined
light of return light from the object to be inspected and reference
light, the return light being obtained by irradiating the object to
be inspected with measuring light, the reference light
corresponding to the measuring light, the optical coherence
tomographic imaging apparatus comprising: an imaging parameter
setting unit for setting an imaging parameter of the tomographic
image of the object to be inspected; an evaluation index selection
unit for selecting an image quality evaluation index in accordance
with the imaging parameter; an image characteristic amount
acquiring unit for acquiring an image characteristic amount from
the tomographic image in accordance with the imaging parameter; and
an image quality evaluating unit for evaluating image quality of
the tomographic image based on the image characteristic amount and
the image quality evaluation index.
2. An optical coherence tomographic imaging apparatus according to
claim 1, further comprising a readjustment unit for readjusting
imaging for the tomographic image based on an evaluation result of
the image quality obtained by the image quality evaluating
unit.
3. An optical coherence tomographic imaging apparatus according to
claim 1, wherein the image characteristic amount comprises a value
based on a histogram which is an occurrence frequency of a signal
value of the tomographic image.
4. An optical coherence tomographic imaging apparatus according to
claim 1, wherein the image quality evaluation index comprises a
plurality of the image quality evaluation indices.
5. An optical coherence tomographic imaging apparatus according to
claim 1, further comprising a setting unit for setting an
evaluation area based on the imaging parameter, wherein the image
characteristic amount is extracted from the set evaluation
area.
6. An optical coherence tomographic imaging apparatus according to
claim 1, wherein the imaging parameter comprises selection of a
reference optical path length.
7. An optical coherence tomographic imaging apparatus according to
claim 1, wherein the imaging parameter comprises selection of which
one of an entire acquired tomographic image and part of the
acquired tomographic image is to be evaluated.
8. An optical coherence tomographic imaging method for acquiring an
image of an object to be inspected by dividing light emitted from a
light source into reference light and measuring light, irradiating
the object to be inspected with the measuring light, and by causing
return light from the object to be inspected to interfere with the
reference light, the optical coherence tomographic imaging method
comprising: setting an imaging parameter of the image; selecting an
image quality evaluation index in accordance with the imaging
parameter; acquiring an image characteristic amount from the
acquired image in accordance with the set imaging parameter; and
evaluating image quality of the image based on the image
characteristic amount and the image quality evaluation index.
9. A program for causing a computer to execute the optical
coherence tomographic imaging method according to claim 8.
10. A storage medium having a computer program stored thereon for
executing the optical coherence tomographic imaging method
according to claim 8.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an optical coherence
tomographic imaging apparatus and an optical coherence tomographic
imaging method. In particular, the present invention relates to an
optical coherence tomographic imaging apparatus and an optical
coherence tomographic imaging method for acquiring a tomographic
image of an eye fundus, a skin, and the like by optical coherence
tomography, and to a program for executing the optical coherence
tomographic imaging method and a storage medium having the program
stored thereon.
[0003] 2. Description of the Related Art
[0004] In recent years, an optical coherence tomographic imaging
apparatus (optical coherence tomography apparatus: hereinafter,
referred to as "OCT apparatus") that employs an optical
interference technology utilizing low-coherent light has been put
into practical use. The OCT apparatus is an apparatus that is
useful in a medical field, in particular, in an ophthalmologic
field. The OCT apparatus is capable of acquiring a tomographic
image of a portion of a retina in an eye fundus, and has therefore
gradually been regarded as an apparatus indispensable for
diagnosing illnesses in an eye fundus portion.
[0005] The principle of the OCT apparatus is briefly described.
Low-coherent light is divided into reference light and measuring
light. The measuring light is caused to enter an object to be
inspected, and return light, which is reflected from an area to be
imaged as a tomographic image, is caused to interfere with the
reference light, with the result that a tomographic image of the
object to be inspected can be acquired. The OCT apparatus is
classified into two types, that is, a time domain (TD) type and a
Fourier domain (FD) type. The FD-OCT apparatus determines a
reflection intensity profile with respect to a distance (depth) by
performing Fourier transform for an acquired interference signal
with respect to a wavenumber. Through scanning of an irradiated
portion of the object to be inspected, the tomographic image can be
acquired. The FD-OCT apparatus can acquire the tomographic image at
higher speed as compared to the TD-OCT apparatus, and hence the
FD-OCT apparatus has become the current mainstream.
[0006] As a method of image quality evaluation for the OCT
tomographic image, for example, there are provided graininess
evaluation using a signal-to-noise ratio (SNR), a contrast-to-noise
ratio (CNR), and a Wiener spectrum, and resolution evaluation using
a modulation transfer function (MTF). Those indices are physical
indices focusing on part of an image quality.
[0007] On the other hand, as an evaluation index reflecting
subjectivity of end users (for example, doctor and clinical
technologist), a quality index (QI) is disclosed in D. M. Stein, J.
G. Fujimoto, et al. Br. J. Ophthalmol. 2006 February; 90(2):
186-190. The QI is an image quality evaluation index determined
based on a histogram for a luminance value of an image, and it is
reported that the QI has a higher correlation with the evaluation
of the end users as compared to the physical indices such as the
SNR.
[0008] FIG. 8 is an illustration of a histogram showing the QI. The
QI is expressed by the following expressions.
QI = IR .times. TSR ( Expression 1.1 ) IR = ( Saturation - low )
Low .times. 100 ( Expression 1.2 ) TSR = Number ( Saturation
.about. Middle ) Number ( Middle .about. Noise ) ( Expression 1.3 )
##EQU00001##
[0009] Note that, in D. M. Stein, J. G. Fujimoto, et al. Br. J.
Ophthalmol. 2006 February; 90(2): 186-190, "Saturation", "Low",
"Noise", and "Middle" are defined as follows.
Saturation: 99th percentile of the histogram Low: 1st percentile of
the histogram Noise: 75th percentile of the histogram Middle:
average value between "Saturation" and "Noise"
[0010] "Number(Saturation.about.Middle)" represents the total
number of pixels having a luminance value in a range of from
"Saturation" to "Middle" in the histogram. "IR" is a term
corresponding to the signal-to-noise ratio (SNR), and "TSR"
represents, as shown in FIG. 8, a ratio of the number of pixels in
a bright layer to the number of pixels in a dark layer.
[0011] Further, as one of the OCT imaging methods in ophthalmology,
imaging using an enhanced depth imaging (hereinafter, referred to
as "EDI") method is known. The EDI method is a method to be used
mainly for observing details of a choroid with a focus on a
relationship between the choroid and the illness. The EDI method is
a method of acquiring a tomographic image as a reverse image under
a state in which a position of a coherence gate is situated more
deeply than the position of the choroid. The coherence gate
represents a position at which the optical distances of the
measuring light and the reference light in the OCT apparatus are
equal to each other.
[0012] In recent years, as described in, for example, IMAMURA,
YUTAKA et al. Retina, Vol. 29, pp. 1469-1473 (2010), studies on the
choroid thickness of affected eyes have been proceeding, and hence
the significance of diagnosing the choroid has been increasing.
[0013] The choroid is a layer which is low in luminance (low in
regular reflectance), and hence, as a portion of the retinal image
which is low in luminance value is rendered with higher accuracy,
the user can diagnose the choroid with higher accuracy. However,
the image quality evaluation index described in D. M. Stein, J. G.
Fujimoto, et al. Br. J. Ophthalmol. 2006 February; 90(2): 186-190
is not appropriate as an index for evaluating the image quality of
an image acquired by the EDI method because the rendering of a
portion of the image which is high in luminance value is rated
high.
SUMMARY OF THE INVENTION
[0014] It is an object of the present invention to perform more
appropriate image quality evaluation.
[0015] Further, it is another object of the present invention to
acquire a more appropriate image through readjustment performed
based on a result of the image quality evaluation.
[0016] According to an exemplary embodiment of the present
invention, there is provided an optical coherence tomographic
imaging apparatus for acquiring a tomographic image of an object to
be inspected based on combined light of return light from the
object to be inspected and reference light, the return light being
obtained by irradiating the object to be inspected with measuring
light, the reference light corresponding to the measuring light,
the optical coherence tomographic imaging apparatus including: an
imaging parameter setting unit for setting an imaging parameter of
the tomographic image of the object to be inspected; an evaluation
index selection unit for selecting an image quality evaluation
index in accordance with the imaging parameter; an image
characteristic amount acquiring unit for acquiring an image
characteristic amount from the tomographic image in accordance with
the imaging parameter; and an image quality evaluating unit for
evaluating image quality of the tomographic image based on the
image characteristic amount and the image quality evaluation
index.
[0017] Further, according to an exemplary embodiment of the present
invention, there is provided an optical coherence tomographic
imaging method for acquiring an image of an object to be inspected
by dividing light emitted from a light source into reference light
and measuring light, irradiating the object to be inspected with
the measuring light, and by causing return light from the object to
be inspected to interfere with the reference light, the optical
coherence tomographic imaging method including: setting an imaging
parameter of the image; selecting an image quality evaluation index
in accordance with the imaging parameter; acquiring an image
characteristic amount from the acquired image in accordance with
the set imaging parameter; and evaluating image quality of the
image based on the image characteristic amount and the image
quality evaluation index.
[0018] According to the present invention, it is possible to
perform more appropriate image quality evaluation through the
selection of the image quality evaluation index in accordance with
the imaging parameter.
[0019] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a flow chart illustrating a first embodiment of
the present invention.
[0021] FIG. 2 is an illustration of an optical coherence
tomographic imaging apparatus according to the first embodiment, a
second embodiment, and a third embodiment of the present
invention.
[0022] FIG. 3 is an illustration of an evaluation area according to
the first embodiment.
[0023] FIG. 4 is an illustration of normal imaging and EDI
according to the second embodiment.
[0024] FIG. 5 is an illustration of histograms acquired in the
normal imaging and the EDI according to the second embodiment.
[0025] FIG. 6 is a flow chart illustrating the second
embodiment.
[0026] FIG. 7 is a flow chart illustrating the third
embodiment.
[0027] FIG. 8 is an illustration of a histogram showing a QI
described in D. M. Stein, J. G. Fujimoto, et al. Br. J. Ophthalmol.
2006 February; 90(2): 186-190.
DESCRIPTION OF THE EMBODIMENTS
[0028] Preferred embodiments of the present invention will now be
described in detail in accordance with the accompanying
drawings.
First Embodiment
[0029] A first embodiment of the present invention is described
with reference to FIGS. 1 to 3.
[0030] FIG. 2 is a configuration diagram of an OCT apparatus
according to an embodiment of the present invention. In FIG. 2, the
OCT apparatus includes a light source 200. In this embodiment, a
super luminescent diode (SLD) light source is used as the light
source 200. However, the light source 200 may be any light source
as long as the light source 200 is a low-coherence light source.
Specifically, an amplified spontaneous emission (ASE) light source,
an ultrashort-pulse light source, such as a titanium-sapphire laser
light source and a super continuum (SC) light source, and a swept
source (SS) light source may be used.
[0031] The OCT apparatus further includes a fiber coupler 201
configured to divide light having a wavelength in a wide band,
which is emitted from the light source 200, into reference light
passing through a fiber 203 serving as a reference optical path and
measuring light passing through a fiber 202 serving as a
measurement optical path. It is desired that the fiber coupler 201
have less wavelength dependency with respect to a division ratio
between the reference light and the measuring light and have a
substantially constant division ratio. The division ratio is set in
consideration of reflectance of a tomographic image of an object to
be inspected, which is acquired by the OCT apparatus. A retina of a
human eye is low in reflectance, and hence it is necessary to
reduce a loss of reflected return light to the extent possible.
Thus, a coupler having a division ratio of 90:10 may be used, but
the present invention is not limited thereto.
[0032] The divided measuring light is output as collimated light
through a fiber collimator 209. Measuring light 210 output as the
collimated light through the fiber collimator 209 irradiates a
retina of an eye 215 that is the object to be inspected. Further,
the measuring light 210 passes through a scanning optical system
constituted by a scanner mirror 211 and a scanner lens 212, for
scanning the retina of the eye 215 with the measuring light 210,
and then focus adjustment is performed by an objective lens 213.
The scanner mirror 211 enables suitable control of scan speed, an
image size, and a scan pattern (vertical scan, horizontal scan,
circle scan, radial scan, or the like) of the OCT apparatus. The
scanner mirror 211 is controlled by a scanner controller 216. The
focus adjustment may be implemented by adjusting a position of an
electrically-driven stage 214. The electrically-driven stage 214 is
controlled by a stage controller 221. The scanner controller 216
and the stage controller 221 are controlled by a PC 220.
[0033] The measuring light 210 is reflected on the retina of the
eye 215, and travels as measuring return light through the
above-mentioned measurement optical path in an opposite
direction.
[0034] On the other hand, divided reference light 208 is output as
collimated light through a fiber collimator 204. For the reference
light 208, dispersion compensation is performed by a dispersion
compensation member 205, and the resultant reference light 208 is
reflected on a reference-system reflection mirror 206. With use of
an optical attenuator (not shown), such as an ND filter, a
mechanism for adjusting light intensity may be provided between the
fiber collimator 204 and the reference-system reflection mirror
206.
[0035] Hereinafter, the dispersion compensation is briefly
described. Various lenses used in the optical system for the
measurement optical path, such as the scanner lens 212 and the
objective lens 213, and vitreous body and a crystalline lens of the
eye 215 that are the object to be inspected each have such a
characteristic that a refractive index thereof differs depending on
the wavelength. Such a characteristic is referred to as dispersion.
It is generally known that vertical resolution of the OCT apparatus
is deteriorated due to the dispersion. When members corresponding
to the various lenses and the vitreous body and the crystalline
lens of the eye 215 are inserted into the reference optical path,
the deterioration in resolution and decrease in interference
intensity can be prevented. Such arrangement and operation are
referred to as the dispersion compensation.
[0036] An electrically-driven stage 207 for adjusting a position of
the reference-system reflection mirror 206 has its position
controlled by the stage controller 221. The movement of the
reference-system reflection mirror 206 using the
electrically-driven stage 207 corresponds to movement of a
coherence gate. The reference light reflected by the
reference-system reflection mirror 206 travels through the
above-mentioned reference optical path in an opposite direction.
Interference light obtained by combining the measuring return light
and the reference light with use of the fiber coupler 201, or
combined light, travels through a fiber 217, and the optical path
thereof is divided by a beam splitter 218 for each wavelength. For
the divided interference light, a photodetector element 219
performs photoelectric conversion for each wavelength, and then the
PC 220 including a data acquisition system performs data
acquisition, imaging, and analysis. The photodetector element 219
is a line sensor camera, and in a case of a wavelength swept light
source such as the SS light source, the beam splitter 218 and the
line sensor camera are not used but a photodiode is used as the
photodetector element.
[0037] A method of imaging data acquired by the photodetector
element 219 is described below. The acquired data is evenly spaced
with respect to the wavelength. On the other hand, in order to
determine a reflection intensity profile with respect to a
distance, Fourier transform is performed with respect to the
wavenumber, and hence the data needs to be evenly spaced with
respect to the wavenumber. Thus, the data acquired by the
photodetector element 219 is subjected to wavelength-wavenumber
conversion, to thereby acquire data evenly spaced with respect to
the wavenumber. The data is subjected to Fourier transform for each
scanning operation, and an absolute value of a complex amplitude
thus output is calculated. In order to display the absolute value
data as an image, logarithmic transformation is performed to
convert the data into a luminance value of 8-bit grayscale. The
luminance value data is matched with positional coordinates of an
image, and is output as two-dimensional image data. Through the
above-mentioned operation, the tomographic image of the retina or
the like that is the object to be inspected is acquired.
[0038] Hereinafter, switching of evaluation indices in accordance
with an evaluation area is described with reference to FIGS. 1 and
3. FIG. 1 is a flow chart illustrating the first embodiment. FIG. 3
is an illustration of the evaluation area and the evaluation index
according to the first embodiment, and the evaluation area and the
evaluation index are output on a monitor included in the PC 220 of
FIG. 2.
[0039] For a subject to be examined for the first time, a subject
who undergoes medical examination or the like and therefore has not
been diagnosed with his/her illness, or a subject whose illness
affects a wide range of the retina displayed in the image, the
entire tomographic image needs to be observed. On the other hand,
for a subject who has been diagnosed with an illness area, there
may be employed an evaluation method in which only the illness area
is evaluated. In this case, the entire retina or the illness area
(for example, detached area or neovascularized area), which is an
anatomical structure to be observed, is set as the evaluation area,
and a tomographic image of the evaluation area is acquired. In Step
101, an input section (not shown) included in the PC 220 inputs the
evaluation area based on an instruction from the user. As
illustrated in FIG. 3, the evaluation area is input by the user
through selection of any one of radio buttons labeled as "retina"
or "illness area" with a device such as a mouse (hereinafter,
referred to also as "setting of imaging parameter" or "imaging
parameter setting step"). The input section functions as an
evaluation area setting unit for setting the evaluation area. The
input section and related components of the PC 220 serve as an
imaging parameter setting unit according to the present invention.
When the evaluation area is set by the user, an evaluation index
(image quality evaluation index) in accordance with the evaluation
area, that is, the imaging parameter, is selected by the PC (Step
102: evaluation index selecting step). The components of the PC 220
for performing the above-mentioned operation correspond to an
evaluation index selection unit according to the present invention.
In a case where the evaluation index in accordance with the
evaluation area is selected from among a plurality of evaluation
indices, the user may select the evaluation index.
[0040] FIG. 3 is an illustration of display screens showing the
setting of the evaluation area and an evaluation result, which are
displayed on the monitor of the PC 220. The upper part of FIG. 3 is
a display example in a case where the entire image ("retina") is
set, and the lower part of FIG. 3 is a display example in a case
where part of the image ("illness area") is set.
[0041] The input section (not shown) included in the PC selects an
image quality evaluation index in accordance with the imaging
parameter that is set based on an instruction from the user.
Specifically, as illustrated in the upper part of FIG. 3, in the
case where the entire retinal image is observed as the evaluation
area, the PC 220 selects an image quality evaluation index "1". On
the other hand, as illustrated in the lower part of FIG. 3, in the
case where only the illness area is set as the evaluation area, the
PC 220 selects an image quality evaluation index "2". That is, the
PC 220 serving as the imaging parameter setting unit according to
the present invention performs an operation of selecting, as the
imaging parameter, whether to set the entire acquired image or part
of the acquired image as the evaluation area.
[0042] The input section (not shown) included in the PC acquires a
tomographic image of the retina so as to include the evaluation
area based on an instruction from the user (Step 103). The
tomographic image may be acquired with use of the OCT apparatus and
the data imaging method as described above. The PC 220 or the user
sets an evaluation index calculation area based on the acquired
image (Step 104). In the case where the entire retinal image is
observed, the PC 220 sets the entire image as the evaluation index
calculation area. On the other hand, in the case where the illness
area is set as the evaluation area, the PC 220 or the user sets an
evaluation index calculation area surrounded by, for example, the
dotted line. Such setting of the evaluation area is executed by the
components of the PC 220 that function as the evaluation area
setting unit. The PC 220 acquires an image characteristic amount
based on the evaluation index calculation area (Step 105: image
characteristic amount acquiring step). A program module for
executing Steps 103 to 105 functions as an image characteristic
amount acquiring unit.
[0043] In this embodiment, a histogram, a luminance value
distribution, or the like is used as the image characteristic
amount. For the image quality evaluation index "1", for example,
the following expressions are used under a condition that the
entire image is set as the evaluation index calculation area.
IQI1=Cont.times.TSR (Expression 2.1)
Cont=(saturation-noise)/2.sup.BitDepth (Expression 2.2)
TSR=(Number(saturation.about.middle)/Number(middle.about.noise))
(Expression 2.3)
where "BitDepth" represents a bit depth of the image, "saturation"
represents a value of the 99th percentile of the histogram, "noise"
represents a value expressed by ((minimum luminance value of
tomographic image of retina)-1), and "middle" represents an average
value between "saturation" and "noise". "Number(saturation-middle)"
represents the total number of pixels having a luminance value in a
range of from "saturation" to "middle" in the histogram. "Cont" is
a term corresponding to a contrast of the tomographic image of the
retina, and "TSR" represents a ratio of the number of pixels in a
bright layer to the number of pixels in a dark layer in the
tomographic image of the retina.
[0044] However, the image quality evaluation index "1" is not
limited to the "IQI1".
[0045] Further, as described above, it is preferred that the image
characteristic amount be represented by the histogram, but for
example, the image characteristic amount may be represented by a
value of a so-called median point obtained based on the histogram.
Further, the histogram also corresponds to a signal value obtained
from each evaluation area in the tomographic image, such as the
luminance value, or an occurrence frequency of a value that is
equal to or larger than the signal value.
[0046] Next, the image quality evaluation index "2" to be selected
in the case where the illness area is set as the evaluation area is
described.
[0047] The evaluation index calculation area is defined as an area
of an arbitrary size including the illness area. The user may set
the evaluation index calculation area as a layered structure of the
displayed tomographic image, or alternatively the PC 220 or the
user may set the evaluation index calculation area as a rectangular
enclosure area in the vicinity of the illness area. In the lower
part of FIG. 3, the evaluation index calculation area is set as the
rectangular area.
[0048] In a case where a plurality of layers (specifically,
external nuclear layer and external limiting membrane layer) are
present in the evaluation area, there may be employed an evaluation
index using a contrast difference between the bright layer and the
dark layer or an SNR. The image characteristic amount is
represented by the luminance value, the luminance value
distribution, or the like. Further, there may be employed a method
in which the image quality evaluation is performed based on the
degree of divergence from a histogram shape in an area of a
previously acquired image, which has the same size and is situated
at the same position. The image characteristic amount is
represented by the histogram, and in this case, in order to compare
the current and previous images, the same area is imaged using a
tracking function or a unit for storing areas imaged by a fundus
camera or the like. Further, the same evaluation index calculation
area as that in the previous image is set, and the current and
previous histograms are compared. It is only necessary that the
degree of divergence be used for the image quality evaluation index
with the previous histogram set as a reference histogram.
Specifically, there is a method in which cumulative histograms are
determined based on the histograms, and the slopes of the
cumulative histograms are compared for indexing. Further, the
evaluation index may be a combination of the above-mentioned
evaluation indices, and this combination of the evaluation indices
may be used for a plurality of independent evaluation indices.
[0049] By the above-mentioned method, the image quality is
evaluated based on the evaluation index and the image
characteristic amount (Step 106). The image quality evaluation
performed in Step 106 is executed by a program module provided in
the PC 220, which functions as an image quality evaluating unit.
After that, the PC 220 illustrated in FIG. 2 stores identification
information of the subject, the acquired image, the selected
evaluation index, and the evaluation result. The evaluation index
for evaluating the image quality under the condition in which the
entire retinal image is set as an evaluation target, and the
evaluation index for evaluating the image quality under the
condition in which only a specific part of the retinal image is set
as an evaluation target are switched in accordance with the
evaluation area, and thus the image quality evaluation can be
performed with higher accuracy.
Second Embodiment
[0050] A second embodiment of the present invention is described
with reference to FIGS. 4 to 6. Note that, in the following
embodiment, the optical coherence tomographic imaging apparatus is
identical with that of the first embodiment, and therefore
description thereof is omitted.
[0051] FIG. 4 is an illustration of normal imaging and EDI as
methods of measurement for the object to be inspected. The normal
imaging and the EDI are distinguished from each other based on
whether the coherence gate is situated at a position 401 above an
object to be inspected 403 or a position 405 below an object to be
inspected 407. A distance from the fiber coupler 201 to the object
to be inspected 403 or 407 in the measurement optical system
(measurement optical system distance) is compared to a distance
from the fiber coupler 201 to the reference-system reflection
mirror 206 in the reference optical system (reference optical
system distance, or reference optical path length). The normal
imaging corresponds to a case where the measurement optical system
distance is larger, and the EDI corresponds to a case where the
reference optical system distance is larger. The left side of FIG.
4 corresponds to the normal imaging, and the right side of FIG. 4
corresponds to the EDI. At the coherence gate, the tomographic
image is folded. As a result, images overlap one on top of another.
The overlapping images cannot be separated from each other, and
hence the coherence gate needs to be set at a position distant from
an observation area in the tomographic image of the object to be
inspected.
[0052] The FD-OCT apparatus has such a characteristic that the
sensitivity near the coherence gate is higher (roll-off
characteristic). From the clinical point of view in which the OCT
apparatus is used for diagnosing the retina, the EDI is mainly used
for focused observation on a lower area of the retina, such as the
choroid. The normal imaging is set as default. In the normal
imaging, a lower area of a choroid 404 may be lost. On the other
hand, in the EDI, a choroid 408 is imaged in a wider range because
the choroid 408 is near the coherence gate. However, there is such
a disadvantage that, when observed, a relatively bright layer, such
as a nerve fiber layer (NFL) or a retinal ganglion cell complex
(GCC) layer, which is situated at an upper portion of the retina
and is distant from the coherence gate, is darker as compared to
the normal imaging due to decrease in sensitivity caused by the
roll-off effect.
[0053] FIG. 5 is an illustration of a representative example of
histograms in the retinal area imaged in the normal imaging and the
EDI. FIG. 5 shows that the EDI is larger than the normal imaging in
number of pixels having a low luminance value. This fact is caused
by, as described above, the difference in visibility among the
choroid, the NFL, and the GCC layer.
[0054] In this embodiment, the image quality evaluation indices are
switched in accordance with the imaging parameter (whether to
select the normal imaging or the EDI). The switching is described
with reference to FIG. 6. The setting of the imaging parameter in
this embodiment corresponds to the setting of the imaging method,
that is, the normal imaging or the EDI (Step 601). In this case,
the switching between the normal imaging and the EDI involves shift
of the coherence gate through the above-mentioned change of the
reference optical path length, and hence the imaging parameter
includes the change or selection of the reference optical path
length. The imaging parameter is set by the user. For example, in
the normal imaging, the evaluation area corresponds to the entire
tomographic image of the retina, and in the EDI, the evaluation
area corresponds to the choroid area.
[0055] Hereinafter, the selection of the image quality evaluation
index in the normal imaging and the EDI (Steps 602 and 603) is
described. Note that, the image quality evaluation index is not
limited to the image quality evaluation index described in this
embodiment, and various image quality evaluation indices may be
employed so as to meet the purpose of imaging.
[0056] In the normal imaging, the evaluation area corresponds to
the entire retinal image, and hence, for the image quality
evaluation index "1", the "IQI1" of the first embodiment is
used.
[0057] On the other hand, the EDI is intended for focused
observation on the choroid in the tomographic image of the retina,
and hence, for the image quality evaluation index "2" (image
quality index 2: hereinafter, referred to as "IQI2"), the following
index is used.
IQI2=Cont.times.Skew (Expression 3)
where "Skew" represents a three-dimensional moment of the histogram
indicating skewness of the retinal area in the histogram. As the
value of the "Skew" is larger, the skewness of the histogram is
larger in the retinal area of the tomographic image (the shape of
the histogram deviates from the normal distribution). As the
choroid layer that is the dark layer is observed more clearly, the
histogram exhibits a larger number of pixels having a low luminance
value, and hence the skewness increases.
[0058] In the above-mentioned manner, the PC 220 selects the image
quality evaluation index in each of the normal imaging and the EDI.
In a case where the evaluation index in accordance with the
evaluation area is selected from among a plurality of evaluation
indices, the user may select the evaluation index. After that, an
interference signal is acquired (Step 604), and the PC 220 forms an
image (Step 605).
[0059] The evaluation index calculation area is set as the entire
image in both the normal imaging and the EDI. In the EDI, the
evaluation index calculation area is set wider to the entire image
instead of the evaluation area corresponding to the choroid, with
the result that evaluation focusing only on the dark area can be
performed with high accuracy. A histogram is acquired as the image
characteristic amount from the acquired image (Step 606), and in
accordance with the selected image quality evaluation index (Step
607), the image quality evaluation is performed based on the
above-mentioned histogram (Step 608 or 609). The image quality
evaluation indices are switched depending on the purpose of
imaging, and thus the image quality evaluation applicable to more
accurate diagnosis can be performed. Also in a case of other
imaging parameters (for example, scan pattern, image resolution,
number of overlapping images, and imaging speed), a suitable image
quality evaluation index may be set through the switching in
accordance with the imaging parameter.
Third Embodiment
[0060] A third embodiment of the present invention is described
with reference to FIG. 7. This embodiment is based on the second
embodiment. In order to obtain a satisfactory result of the
evaluation using the image quality evaluation index so as to
enhance the image quality, in this embodiment, there is provided a
feedback configuration for the OCT apparatus, which employs an
image quality evaluation index and a reference value.
[0061] Whether to perform the feedback is determined based on
whether or not the image quality evaluation index exceeds the
reference value (Step 710). The feedback is performed when it is
determined that the image quality evaluation index does not exceed
the reference value. In the condition of Step 710, when the image
quality evaluation index does not exceed the reference value,
readjustment (Step 711) is performed before the acquisition of the
interference signal. This operation is executed by the components
of the PC 220 that function as a readjustment unit. After the
readjustment in Step 711, the PC 220 forms an image and determines
a histogram in Steps 704 to 706 in a manner similar to the above.
After that, in accordance with the selected image quality
evaluation index (Step 707), the image quality evaluation is
performed based on the above-mentioned histogram (Step 708 or 709).
After that, the operation proceeds to Step 710, in which the result
of the evaluation using the image quality evaluation index is
compared to the reference value, and when the image quality
evaluation index exceeds the reference value, the operation is
finished.
[0062] When the value of the "IQI1" in the normal imaging is low,
the following reasons are conceivable as an example.
[0063] The contrast of the tomographic image of the retina is
low.
[0064] The position of the coherence gate is distant from the
retinal image.
[0065] Therefore, as the readjustment, focus adjustment or
adjustment of the position of the coherence gate is performed, and
thus a satisfactory result of the image quality evaluation can be
obtained. With the "IQI1" and the feedback configuration, the
luminance value and contrast of the entire tomographic image of the
retina can be increased. Specifically, a histogram obtained in a
case of unsatisfactory imaging for the tomographic image of the
retina exhibits a symmetric distribution with a small distribution
width across a peak value. As the readjustment, focus adjustment
(corresponding to movement of the objective lens 213 of FIG. 2) or
reflectance adjustment (adjustment of the optical attenuator (not
shown), such as an ND filter, between the fiber collimator 204 and
the reference-system reflection mirror 206) is performed, and thus
the value of the "Cont" part of the "IQI1" is mainly increased,
with the result that the image quality is enhanced and the value of
the image quality evaluation index "IQI1" is increased. Further,
the position of the reference-system reflection mirror 206 is
adjusted so as to bring the position of the coherence gate closer
to the layer area of the tomographic image of the retina to the
extent that the coherence gate is not situated thereon. As a
result, the decrease in sensitivity caused by the roll-off can be
prevented. Note that, the above-mentioned operations are identical
with the adjustment performed to some extent before the acquisition
of the interference signal in Step 704 and the readjustment in Step
711 of FIG. 7, and are fine adjustment performed in consideration
of the selected image quality evaluation index and the image
quality evaluation.
[0066] As a result, the NFL and the GCC layer that are the bright
layers near the coherence gate become brighter. Further, the image
quality is enhanced and the value of the "IQI1" is increased along
with the increase in values of the "Cont" and "TSR".
[0067] When the value of the "IQI2" in the EDI focusing on the
choroid layer is low, the following reasons are conceivable as an
example.
[0068] The contrast of the tomographic image of the retina is
low.
[0069] The choroid layer is distant from the coherence gate, or the
coherence gate is situated on the choroid layer so that the image
is folded in the choroid area.
[0070] Therefore, as the feedback configuration, the readjustment,
such as focus adjustment or adjustment of the position of the
coherence gate, is performed, and thus the choroid layer can be
observed with higher accuracy for the thickness and luminance
distribution. In the EDI intended for imaging of the choroid, the
choroid layer can be imaged with high precision when using the
image quality evaluation index and feedback configuration suitable
for the EDI, with the result that the image quality of the
tomographic image of the retina is enhanced.
[0071] As described above, according to this embodiment, in the
case of the unsatisfactory result of the evaluation, appropriate
readjustment can further be performed in accordance with the
imaging parameter.
Other Embodiments
[0072] Further, the present invention is also implemented by
executing the following processing. Specifically, in this
processing, software (program) for implementing the functions of
the above-mentioned embodiments is supplied to a system or an
apparatus via a network or various kinds of storage medium, and a
computer (or CPU, MPU, etc.) of the system or the apparatus reads
and executes the program.
[0073] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0074] This application claims the benefit of Japanese Patent
Application No. 2011-143919, filed Jun. 29, 2011, which is hereby
incorporated by reference herein in its entirety.
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